Communicating between the analog and digital worlds requires devices that can translate the language of the two worlds. A digital-to-analog (D/A) converter accepts adigital word as its input and translates, or converts,this word to an analog voltage. Digital to analog converters (DAC) are components widely used in data processingsystems. The basic elements of many DACs are a resistornetwork, current or voltage switches, a reference supply,and an output operational amplifier.
FIG. 1 shows the structure of a conventional DAC. The basic converter is enclosed within the dashed rectangle(110). It comprises digitally operated switches (112)connected to a resistor network (114) to which is appliedan analog reference voltage Vref. The resistor network 114generates an output current. The basic converter receivesinputs from an input storage register (100) having digitalinputs u(i=1 . . . n) of different weights and an input latchsignal. Each digital input u(i=1 . . . n) is equal to 1 or 0. The output current of the basic converter 110 is then connected to an operational amplifier which has a feedback resistor. The output voltage E.sub.o of said amplifier depends on the analog reference voltage Vref, the values of the digital inputs u(i) according to their weight. Instead ofusing a voltage reference, a current reference may be used as will be described later on in the present invention.
The DAC circuitry involves a voltage or current reference,a resistive ladder network that derives weighted currents or voltages, usually as discrete fractions of the reference, and a set of switches, operated by the digital in puts, that determine which currents or voltages will be summed to constitute the output. The output of the DAC is proportional to the product of the digital input value and the reference. In many applications, the reference is fixed, and the output bears a fixed proportion to the digital input. In other applications, the reference, as well as the digital input, can vary; a DAC that is used in these applications is thus called a multiplying DAC. It is principally used for imparting a digitally controlled scale factor, or gain to analog input signal applied at the reference terminal.
Therefore, a DAC is often used for multiplying a sampled digital input by another analog and continuously variable input. It also has a built-in function which is to sequence the samples with a regular interval.
In the article "A charge-transfer multiplying DAC" by Jos. F. Albarran from the IEEE journal vol. SC 11, pp. 772-779, Dec 1976, a new charge-transfer multiplying DAC employs an array of binary-weighted MOS capacitors and MOS resistors as its only elements. The DAC can be fabricated on the same chip and by the same process and provides two or four quadrant multiplication. An experimental n-channel metal-gate MOS realization demonstrated accuracy to 7 bits plus sign, and a 200 kHz bandwidth. The frequency of this implementation is not high enough to be used in the present invention because it should reach at least 10 MHz. This article discloses a circuit, refer to FIG. 2-A, that contains a control logic circuit (200) receiving inputs (B1, . . . BN), a clock circuit (210), a plurality of switches (220) and an integrated portion circuit (230) which comprises a N-bit binary weighted capacitor array, and transistors. The integrator uses a FET-input operational amplifier and its output is sampled. This DAC may be used as a multiplying DAC in the present invention but cannot be a substitute for the whole device of the present implementation.
The high frequency spectrum of a conventional converter can be attributed to the abrupt changes of the converter output when the digital input value changes. For this reason, a conventional DAC is always followed by a more or less complex low pass filter, the task of which is to smooth out the converter output (FIG. 2-B).
Whereas at low frequencies, techniques already exist that allow such a filter to be implemented in silicon, it is no longer the case at high frequencies.